1. Technical Field of the Invention
The present invention relates generally to the telecommunications field and, in particular, to the use of adaptive antenna arrays or distributed antennas in a forward link of a Direct Sequence-Code Division Multiple Access (DS-CDMA) cellular communications system.
2. Description of Related Art
The next generation of cellular communications systems will be required to provide a broad range of services including digital voice, video and data in different transmission modes. These systems will require higher bit rates and higher received signal power levels, which will result in increased interference between users. Consequently, in order to obtain the high capacities that will be required of these systems, the interference levels will have to be reduced dramatically, and especially in the forward links (network to mobile direction).
Reduced user interference levels in the forward links of these systems can be obtained by increased base station antenna sectorization. Adaptive antenna arrays can be utilized to form relatively narrow beams and thus reduce the size of the user interference areas. For example, adaptive or steerable antenna arrays can provide high antenna gains by transmitting information to individual users in highly directional, narrow beams or lobes. These narrow beams reduce the areas of potential interference. Similarly, distributed antenna systems utilize a plurality of antenna elements positioned at different locations (e.g., "radio access ports"). Consequently, information is transmitted to a user from the closest antenna element (port), which also serves to reduce the potential areas of user interference.
In such a sectorized-antenna CDMA system, base station transmits the traffic channel data on a code channel for a particular mobile station in one antenna lobe. If pilot channel is broadcast over the same area as the narrow antenna lobes, then each antenna lobe is treated as a separate cell. Consequently, if the mobile station moves into a different antenna lobe, which defines a different cell, a handoff of the mobile station between the two cells will have to occur. With each pilot channel thus defining a cell, increased antenna sectorization results in a situation where the mobile stations cross the cell borders more frequently, which leads to a larger number of handoffs. Since there is an upper limit to the number of handoffs such a system can process efficiently, a design trade-off must be made between the amount of antenna sectorization desired and user interference that can be accepted.
Current DS-CDMA systems being developed include those that adhere to the IS-95 standard (ANSI J-STD-008) and the European RACE Project R2020 (known as the Code Division Testbed or "CODIT"). Both of these types of systems use a broadcasted pilot channel in the forward link for two main purposes (once the initial connection between a mobile and base station has been established): (1) identifying individual cells for Mobile-Assisted Handoffs (MAHOs); and (2) facilitating coherent detection of traffic channel data by the mobile stations. Essentially, these systems will use the same pilot symbols for broadcasted cell identification information and facilitating coherent detection by all mobiles in a cell, in order to improve system performance without adding excessive pilot symbol overhead.
For such IS-95 and CODIT systems, the forward link air interfaces have been specified so that the mobile stations are required to use QPSK modulation with coherent detection. The pilot channel is used to facilitate the coherent detection process. Consequently, if the traffic channel is transmitted over a different (adaptive) antenna lobe or from a different (distributed) antenna element than the pilot channel, the mobile stations will experience significant detection errors. These detection errors can occur for two reasons: (1) the searching algorithm used in the mobile stations will direct the receivers to demodulate rays that contain pilot signal energy but no traffic signal energy; and (2) the phase of the pilot channel and traffic channel will be different, because they are transmitted from different antennas or different sets of antenna elements. Consequently, these conditions cause the mobile stations to make erroneous channel estimations which lead to corrupted demodulated signals.
Relatively simple approaches to resolving these problems would be to use either non-coherent detection by the mobile station in the forward link, or provide dedicated pilot symbols for each traffic channel. However, the performance of conventional systems using those approaches would be degraded substantially, in comparison to systems using coherent detection (at the mobile station) facilitated by broadcasting a common pilot channel. Also, newer systems using these approaches would not be compatible with those conventional mobile stations that are designed to use the broadcasted pilot channel to detect and demodulate incoming signals.